1. Field of the Invention
The present invention relates to a heat-sensitive stencil sheet, and more particularly, to a heat-sensitive stencil sheet comprising a thermoplastic resin film of constant surface smoothness laminated on a porous support even when the sheet is wound up into a roll or a plurality of sheets are stacked for a long period of time, and which provides excellent printing images.
2. Background Art
A heat-sensitive stencil sheet is produced, for example, by laminating a thermoplastic resin film having a thickness of approximately 2 xcexcm, directly or via an adhesive, to an ink-permeable porous support such as Washi (Japanese paper), and applying a releasing agent to the surface of the film to prevent sticking.
In order to make a stencil master, a thermal printing head (TPH) is typically employed. In this case, if the surface of the stencil sheet is rough, uniform contact cannot be attained between the TPH and a film laminated on a porous support, resulting in some parts being easily perforated and other parts being difficult to perforate. Accordingly, the resultant perforations do not necessarily reflect the original text or drawing faithfully, and satisfactorily printing images cannot be obtained.
To avoid this shortcoming, attempts have been made to secure surface smoothness of a stencil sheet by use of a porous support having excellent surface smoothness. Particularly, in recent years, stencil sheet rolls, i.e., stencil sheets wound up into a roll around a core such as a paper tube, are often employed so as to enhance operation efficiency of stencil printing. In this case, even though the stencil sheet has excellent surface smoothness when assuming the form of a flat sheet, winding pressure applied to the sheet during winding to form a roll deteriorates surface smoothness with the passage of time, to thereby fail to obtain excellent printing images.
In order to solve the above problem, Japanese Patent Application Laid-Open (kokai) No. 6-239048 discloses a method for preventing deterioration of the surface smoothness of a stencil sheet by adjusting the winding density of a roll. However, even though winding density is controlled, pressure is applied to a stencil sheet when the sheet is pulled from the roll set in a stencil printing apparatus, to thereby cause xe2x80x9ctight windingxe2x80x9d of the stencil sheet roll, with the result that surface smoothness of the stencil sheet cannot be maintained, particularly in the vicinity of the core of the roll. If winding pressure is reduced so as to avoid this drawback, another problem is caused in that handling of the roll becomes poor due to phenomena such as xe2x80x9ccore separationxe2x80x9d, xe2x80x9cdishing,xe2x80x9d and xe2x80x9ctelescoping.xe2x80x9d
As has been conventionally performed, when a porous support is directly subjected to measurement of surface smoothness by use of an air-leak tester such as an Ohken smoothness tester or a PPS (Parker Print Surf) smoothness tester, the smoothness cannot be measured correctly, due to leakage of air through the pores, and measured smoothness values disadvantageously vary widely. In addition, a laser surface profile tester cannot measure surface smoothness correctly, since the measured values are affected by the thickness of the porous support.
Japanese Patent Application Laid-Open (kokai) No. 8-67081 discloses that surface roughness of a thermoplastic resin film laminated onto a porous support is measured by use of surface roughness tester (Model SE-3E, product of Kosaka Seisakusho, stylus surface roughness tester) in accordance with JIS B-06101. The publication has proposed a method for improving performance in perforation of a stencil sheet by use of a thermal head, through the selection of a porous support having a surface roughness of 10 xcexcm or less based on the above measurement. However, since the surface smoothness of the porous support of a stencil sheet is not specified, deterioration of surface smoothness cannot be prevented if the stencil sheet is wound up into a roll or stored in a stacked state for a long period of time. In addition, according to this prior art method, measurement of surface smoothness is performed after production of a stencil sheet by heat-pressing a porous support and a film. Therefore, surface smoothness of the produced sheet product cannot be estimated before production of the stencil sheet.
In view of the foregoing, the present inventors have conducted earnest studies on the mechanism of deterioration of surface smoothness of stencil sheets stored in a rolled state or in a stacked state for a long time, and have found that the above-mentioned problems can be removed by correctly measuring, without large variation, surface smoothness of the porous support of a heat-sensitive stencil sheet and by specifying and determining the surface roughness. The present invention was achieved based on this finding.
Accordingly, an object of the present invention is to provide a heat-sensitive stencil sheet which comprises a thermoplastic resin film of constant surface smoothness laminated on a porous support even when the sheet is wound up into a roll or a plurality of sheets are stacked for a long period of time, and which provides excellent printing images by preventing deterioration of characteristics related to perforation by a thermal head.
Accordingly, in a first aspect of the present invention, there is provided a heat-sensitive stencil sheet comprising a thermoplastic resin film laminated onto a porous support, in which the PPS surface smoothness of the surface of the porous support of the stencil sheet is 1.2 xcexcm or less as measured while a film having PPS surface smoothness of 0.0 xcexcm and a thickness of 0.1-10 xcexcm is pressed against the surface of the porous support.
Preferably, in the first aspect of the invention, the PPS surface smoothness of another surface of the porous support to be laminated with a thermoplastic resin film is 0.9 xcexcm or less as measured while a film having a PPS surface smoothness of 0.0 xcexcm and a thickness of 0.1-10 xcexcm is pressed against the other surface of the porous support.
Preferably, the porous support has an average fiber diameter of 2-20 xcexcm.
In stencil sheet rolls and stencil sheets in a stacked state, the film surface of the stencil sheets adheres to a surface of a porous support of other stencil sheets due to tension during winding or pressure during storage or transportation. The adhesion causes transfer of the rough profile of the surface of the porous support to the film surface of the contacting stencil sheet. For example, an unsupported part of the film on a porous support is dented due to pressure from stacked stencil sheets as time passes, to thereby deteriorate surface smoothness of the film. Therefore, time-elapsed deterioration of surface smoothness of the film surface of a stencil sheet can be prevented by providing a porous support having a smooth surface which maintains contact with other film surface of a stencil sheet.
In the present invention, PPS surface smoothness of the surface of a porous support that maintains contact with the film surface of a stencil sheet is measured while a film having a PPS surface smoothness of 0.0 xcexcm and a thickness of 0.1-10 xcexcm is pressed against the surface of the porous support. Therefore, air leakage in the thickness direction of the porous support is eliminated, to thereby enable correct measurement of surface smoothness of the surface of the porous support of a stencil sheet without large variation. Furthermore, the above measurement enables selection of a porous support having a PPS surface smoothness of 1.2 xcexcm or less, so as to surely and effectively prevent time-elapsed deterioration of surface smoothness of the film surface of stencil sheets.
In the present invention, a PPS surface smoothness of the porous support surface of a heat-sensitive stencil sheet is measured while a film having a PPS surface smoothness of 0.0 xcexcm and a thickness of 0.1-10 xcexcm, preferably 0.1-5.0 xcexcm, more preferably 0.1-3.0 xcexcm, is pressed against the surface of the porous support. The thus-measured surface smoothness is 1.2 xcexcm or less, preferably 1.0 xcexcm or less, more preferably 0.8 xcexcm or less.
When the film for measurement does not have a PPS surface smoothness of 0.0 xcexcm and the thickness of the film falls out of the range of 0.1-10 xcexcm, the surface profile of a porous support is not reproduced correctly, to fail to measure surface smoothness of the porous support correctly.
In practice, a thermoplastic resin film which is actually applied to a stencil sheet is preferably used as the film which is pressed against a support during measurement. The term xe2x80x9cPPS surface smoothnessxe2x80x9d refers to surface smoothness measured by an air-leak method used a Parker Print Surf smoothness tester (product of Messmer Buchel Co.).
When the PPS surface smoothness of the porous support surface of a heat-sensitive stencil sheet is in excess of 1.2 xcexcm, there cannot be prevented time-elapsed deterioration of surface smoothness of the film surface of a stencil sheet that maintains contact with a porous support when the stencil sheet is in a stacked state or wound into a roll.
In addition, the PPS surface smoothness of the surface of another porous support to be laminated with a thermoplastic resin film is 0.9 xcexcm or less, preferably 0.6 xcexcm or less as measured while a similar film for measurement as described above is pressed against the other surface of the porous support. When the PPS surface smoothness is in excess of 0.9 xcexcm, a heat-sensitive stencil sheet made by lamination of the porous support and a thermoplastic resin film is likely to have poor surface smoothness of the film surface.
The porous support preferably has an average fiber diameter of 2-20 xcexcm, more preferably 3-15 xcexcm. An average fiber diameter in excess of 20 xcexcm may result in a PPS surface smoothness of greater than 1.2 xcexcm as measured while a film is pressed against a porous support, whereas an average fiber diameter of less than 2 xcexcm may result in failure to maintain fiber density of the porous support such that ink-permeability is not impeded.
The heat-sensitive stencil sheet is prepared by lamination of a thermoplastic resin film and a porous support.
No particular limitation is imposed on the resin of the thermoplastic resin films used in the present invention. Examples of the resins include polyester, polyamide, polypropylene, polyethylene, poly(vinyl chloride), poly(vinylidene chloride), and other known polymers. Although such films having a small thickness are advantageous in view of perforation sensitivity, production of thin films becomes more expensive. Therefore, the material of the film is preferably selected in accordance with characteristics of the material during thermal perforation by use of a thermal head, such as film thickness, melting point, heat shrinkage (percentage), and shrinkage stress. Of these, stretched polyester films are particularly preferred in view of perforation sensitivity.
Examples of polyesters formed into a polyester film include polyethylene terephthalate, ethylene glycol-terephthalic acid-isophthalic acid copolymers, poly(ethylene 2,6-naphthalate), poly(hexamethylene terephthalate), and hexamethylene glycol-1,4-cyclohexanedimethylene glycol-terephthalic acid copolymers.
The above thermoplastic resin films typically have a thickness of 0.1-10 xcexcm, preferably 0.1-5.0 xcexcm, more preferably 0.1-3.0 xcexcm. When the thickness is in excess of 10 xcexcm, suitability for perforation may be deteriorated, whereas when the thickness is less than 0.1 xcexcm, stability of the formed films may be deteriorated.
A variety of additives may be incorporated into the thermoplastic resin films in accordance with needs. Examples of the additives include flame-retardant agents; heat stabilizers; anti-oxidants; UV-absorbers; anti-static agents; pigments; dyes; organic lubricants such as fatty acid esters and waxes; and anti-foaming agents such as polysiloxane.
No particular limitation is imposed on the porous supports used in the present invention so long as they are porous materials which cannot be perforated substantially during heat from a thermal head and can permeate ink during a printing process. For example, porous supports such as tissue papers, machine made papers, non-woven fabrics, fabrics, and screen cloths may be employed. The tissue papers or machine made papers may be made of natural fibers such as Manila hemp, paper mulberry, Mitsumata (Edgeworthia papyrifera) pulp; or synthetic fibers such as polyester fibers, vinylon, nylon fibers, and rayon. These fibers may be used singly or in combination of two or more species.
The area ratio of pores at a surface of a porous support (pore ratio) obtained by observation of a surface plane of the support is preferably 5-80%, more preferably 5-50%, particularly preferably 5-30%. When the pore ratio is less than 5%, permeability of ink is poor, and images are incompletely printed to thereby lower clearness of the images. When the pore ratio is in excess of 80%, permeability of ink increases to provide a bleeding image, and strike-through occurs easily. The term xe2x80x9cpore ratioxe2x80x9d refers to the percentage of the area of pores in relation to a specific surface area of a support, as obtained by observation of a surface plane of the support.
No particular limitation is imposed on the method of lamination of a thermoplastic resin film and a porous support, and any method is acceptable so long as the method causes no delamination in a routine operational state and no impediment to perforation of the film and permeation of ink. Typically, an adhesive is employed in lamination, while a thermoplastic resin film may be melt-bonded to a support made of synthetic fiber.
Examples of ingredients of the adhesive include vinyl acetate, acrylic compounds, vinyl chloride-vinyl acetate copolymers, polyesters, and urethanes. Combinations of polyester acrylates, urethane acrylates, epoxy acrylates, or polyol acrylates and a photopolymerizati on initiator may also be employed as a UV-curable adhesive.
In this case, an adhesive predominantly containing urethane acrylate is particularly preferred. The adhesives may further contain other additives, such as antistatic agents and lubricants, in accordance with needs.
In order to prevent sticking to a thermal head and other objects, the heat-sensitive stencil sheet of the present invention preferably has, on the thermoplastic resin film, an anti-sticking layer containing a releasing agent. Known parting agents such as silicone oil, silicone resins, fluororesins, and surfactants may be used. In addition, additives such as antistatic agents, heat-resisting agents, antioxidants, organic particles, inorganic particles, and pigments may be incorporated into an anti-sticking layer in an amount within the range in which the effect of a releasing agent is not impeded. Furthermore, to a coating material of the anti-sticking layer, additives such as dispersing aids, surfactants, antiseptic agents, and defoaming agents may be added in order to enhance water-dispersibility of the above-described additives. The anti-sticking layer typically has a thickness of 0.005-0.4 xcexcm, preferably 0.01-0.4 xcexcm, in view of a running property of a thermal head during perforation and prevention of staining of the head. When the parting agent and other additives are used, they are preferably dissolved in water or emulsified or suspended, in view of environmental safety during an application step and effects on the human body.